NStar's agreement to buy 27.5 percent of the wind farm's power is similar to National Grid's agreement with Cape Wind to buy half its power, which the Department of Public Utilities approved in 2010. The $2.6 billion Cape Wind project still is seeking financing, and company officials said having two large power purchase deals in hand will give them a boost.

The 130-turbine project in Nantucket Sound, proposed in 2001, is aiming to be the country's first offshore wind farm, and hopes to produce power by late 2015. It's backed by Massachusetts Gov. Deval Patrick, but staunchly opposed by critics who have filed pending lawsuits and argue its power is too costly. For instance, under its 15-year contract, NStar has agreed to buy Cape Wind's power for more than double what conventional energy is projected to cost during the same period.

"It's disappointing the state is willing to burden Massachusetts households and businesses with billions of dollars in extra utility costs for the sake of promoting an expensive and poorly sited project," said Audra Parker of the Alliance to Protect Nantucket Sound, the project's chief opponent.